This invention provides a novel catalyst and method for the formation of amide bonds. The catalysts are derivatives of hydroxybenzotriazole (HOBT) and carry a positive charge at pH 5-7. These reagents are useful for forming compounds containing an amide bond with high purity and yield.
A wealth of procedures are available for forming amide bonds. Nonetheless, there is still room for improvement. For example, procedures have been developed which use reagents such as carbodiimides as amide coupling agents. These carbodiimides include dicyclohexylcarbodiimide (DCC), diisopropylcarbodiimide (DIC), and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC). Unfortunately, these particular reagents can require a large excess of acid, can be problematic for difficult couplings with unreactive carboxylic acids and/or amines and can be difficult to purify from the reaction mixture.
Catalysts such as 1-hydroxybenzotriazole (HOBT) and derivatives thereof have also been used. 
Unfortunately, HOBT and its derivatives can be difficult to remove from the reaction media.
Briefly stated, the invention is a catalyst, the use of the catalyst and a method of using the catalyst in the formation of a compound with an amide bond. The catalyst is an HOBT derivative having the following formula (1): 
where R1 is a group bearing a positive charge at pH 5-7; where Y is a bond or a substituted or unsubstituted alkylene chain containing 1-10 carbon atoms and 0-2 heteroatoms selected from the group consisting of N, S and O; and where X is a linker group selected from xe2x80x94COxe2x80x94 or xe2x80x94SO2xe2x80x94.
In accordance with the method aspect of the invention, an amine, a carboxylic acid, an amide coupling agent and a catalyst of formula (1) are reacted for a time sufficient to produce an amide bond between the amine and the carboxylic acid. Thereafter, the compound containing the amide bond is isolated.
In accordance with a preferred method, the reaction is carried out in solution. Also, the amide coupling agent is a carbodiimide, which carbodiimide bears a positive charge at pH 5-7. The catalyst is most preferably PP-HOBT (see formula (2) below). Finally, the compound containing the amide bond preferably does not bear a positive charge at pH 5-7 and is isolated by passing the entire reaction mixture through a cation exchange resin which traps unwanted materials and allows the desired amide product to be isolated from the eluent with excellent purity.
This preferred method, wherein the amide coupling agent and catalyst both carry a positive charge at pH 5-7, provides the advantage that the compound containing the amide bond can be readily isolated from the reaction mixture when the mixture is passed through a cation exchange resin.
As used herein and the appended claims, the term xe2x80x9ccatalystxe2x80x9d is intended to have a relatively broad meaning referring to a compound that facilitates the reaction between one or more other compounds, the catalyst remaining in or returning to its original state.
As used herein and the appended claims, xe2x80x9calkylxe2x80x9d means an unbranched or branched aliphatic or cyclic group containing 1-20, preferably 1-10, carbon atoms.
As used herein and the appended claims, xe2x80x9calkylenexe2x80x9d means an unbranched or branched carbon chain containing 1-20, preferably 1-10, carbon atoms. Branched carbon chains include both linear and cyclic structures.
As used herein and the appended claims, xe2x80x9carylxe2x80x9d means a mono- or polycyclic carbocyclic aromatic ring containing 6 to 10 carbon atoms, such as phenyl (Ph) or naphthyl. When more than 1 ring is present, the rings can be fused or unfused.
As used herein and the appended claims, xe2x80x9cheteroarylxe2x80x9d is a mono-, bi- or tricyclic, N-, O- or S-heteroaryl, such as benzofuran, benzothiophene, furan, imidazole, indole, isothiazole, oxazole, piperazine, pyrazine, pyrazole, pyridazine, pyridine, pyrimidine, pyrrole, quinoline, thiazole, and thiophene.
The catalysts of the present invention have the general formula (1): 
where R1 is a group bearing a positive charge at pH 5-7;
Y is a bond or a substituted or unsubstituted alkylene chain containing 1-10 carbon atoms, and, optionally 1 or more heteroatoms such as N, S or O; and
X is a linker group selected from xe2x80x94COxe2x80x94 or xe2x80x94SO2xe2x80x94.
The function of R1 is to allow the catalyst of the present invention to be removed by a cation exchange resin; thereby allowing the formed compound containing the amide bond to be purified via ion exchange chromatography. R1 can be any group bearing a positive charge at pH 5-7. R1 is preferably a substituted or unsubstituted, aliphatic or cyclic, dialkylamine (such that R1 is a tertiary amine in formula (1)). Preferably, R1 is a cyclic dialkylamine such as piperidine. Most preferably, Y is a bond and R1 is 4-piperidino-piperidine.
Y is preferably a substituted or unsubstituted alkylene chain containing 1-10 carbon atoms.
The substituents for R1 and Y can be selected for optimization of the solubility characteristics of the catalyst in various solvents. For example, as the number of carbon atoms in the Y substituent increases, the solubility of the catalyst in nonpolar solvents is expected to improve. Alternatively, groups such as ethers and the like can be added to the R1 or Y to enhance solubility in polar solvents.
Suitable substituents for R1 and Y include groups which are relatively unreactive with carboxylate groups and amine groups. Examples include C1-6 alkyl, C1-6 alkoxy, and halogen. Amines and carboxylates are not typically suitable substituents as this would cause the catalyst to react with the starting materials, resulting in undesired side products. Presently, xe2x80x94SO2xe2x80x94 is preferred as the linker group X.
The most preferred embodiment of the present invention is a catalyst of the formula (2): 
This particular catalyst has acceptable solubility in DMF and can be readily removed from the reaction mixture by passing the mixture through a cation exchange resin.
Preparation of the Catalyst
This most preferred catalyst, which includes a sulphonamide linker group X, may be synthesized in accordance with the following: 
The chemistry depicted in this scheme is derived from Pop et al. (J. Org. Chem. 1997, 62:2594, the disclosure of which is incorporated herein by reference), who described the preparation of a resin bound HOBT.
When X is a carbonyl group, the catalysts may be prepared from 6-acyl chloride-HOBT as shown below: 
Method of Amide Bond Formation
The invention provides a method of forming a compound containing an amide bond. The steps comprise reacting an amine, a carboxylic acid, a coupling agent and a compound of the general formula (1) described above. The general reaction is represented by the following: 
The reaction is carried out under conditions and for a time sufficient to produce an amide bond between the amine and the carboxylic acid.
Suitable amines (R1) include any compounds containing a free primary or secondary amine, including, for example, substituted or unsubstituted alkylamines, dialkylamines, arylamines and heteroarylamines. As is commonly done, HCl salts or other salts of the amines can also be used as the starting materials.
Use of the catalyst of the invention provides an advantage when the amine used in the amide formation reaction is provided in the form of an HCl salt. Typically, when using an HCl salt of an amine, a separate base is added to the reaction mixture to liberate the amine. However, the present catalyst, particularly with the tertiary amine in the R1 group, can fill this role and eliminate the necessity to add a separate base to the reaction mixture.
Suitable carboxylic acids (R2xe2x80x94CO2H) include any compounds containing a free carboxylate, including, for example, substituted or unsubstituted alkylcarboxylates, arylcarboxylates or heteroarylcarboxylates.
The above amines and carboxylates can be substituted 1 or more times (preferably 1 to 3 times) with a halogen, hydroxyl, amine, alkylamine, dialkylamine, alkyl, alkoxy, alkenyl, alkylcarboxylate, thiol, and alkylcarboxyamide. In addition, the starting amines and carboxylic acids may have protected functional groups.
In one aspect of this invention, the catalyst of the present invention is used in the formation of peptide bonds between a first amino acid containing moiety and a second amino acid containing moiety. Each of these amino acid containing moieties can include one or more amino acids. Suitable amino acids include naturally encoded amino acid residues, unusual or unnatural amino acid residues, and synthetic amino acid residues.
Preferably, the compound containing an amide bond produced by the method of the present invention is one that does not bear a positive charge at pH 5-7. In this way, the compound can be isolated from the catalyst by passing the reaction mixture through a cation exchange resin.
Typically, amide coupling agents are compounds that function as dehydrating agents, thus driving the condensation reaction between a carboxylic add and an amine. Preferably, the amide coupling agent is a compound that bears a positive charge at pH 5-7. In this way, the amide coupling agent may be removed from the reaction solution by the same cation exchange resin as that used for the catalyst. Alternatively, the coupling agent does not bear a positive charge and other means are used to remove it from the reaction mixture. For example, precipitation of the byproduct of the coupling agent. Moreover, other conventional chromatographic means may be used to remove the amide coupling agent.
Suitable amide coupling agents include benzotriazolyloxy-tris(dimethylamino) phosphonium Hexafluorophosphate (BOP) and carbodiimides such as dicyclohexylcarbodiimide (DCC), diisopropylcarbodiimide (DIC), and 1-(3-dimethylaminopropyl)-3ethylcarbodiimide hydrochloride (EDC). EDC is the most preferred carbodiimide to use as the amide coupling agent.
Preferably, the reaction is carried out in solution. This is particularly advantageous because the catalyst, and preferably the amide coupling agent as well, can be removed from the reaction solution by passing the solution through a cation exchange resin. Alternatively, the reaction is carried out on solid phase, for example, a solid phase peptide synthesis such as that described in U.S. Pat. No. 4,888,385, the disclosure of which is incorporated herein by reference.
Suitable solvents for the reaction include non-nucleophilic organic solvents, such as dimethylformamide (DMF), dimethylsulfoxide (DMSO), methylene chloride (DCM or CH2Cl2) and the like.
The reaction is typically conducted for at least 30 minutes to 24 hours. Preferably, the reaction is conducted for about 12 hours.
The reaction is typically conducted at room temperature (about 25xc2x0 C.), although any reaction temperature conducive to the reaction can be used.
The compound containing the amide bond can be isolated using conventional purification means, including, for example, extraction, chromatography, crystallization, etc. However, the preferred method of isolation involves passing the reaction mixture through a cation exchange resin. Preferred cation exchange resins include silica based resin substituted with functional groups, including SCX (benzensulfonic acid resin), PRS (propylsulfonic acid resin) and CBA (carboxylic acid resin); all of which are available from Varian Sample Preparation (Harbor City, Calif.). The compound containing the amide bond will wash through the column, while the catalyst, and preferably the amide coupling agent as well, will bind to the column. Preferably, about 1 g of resin is used per 0.7 to 1 eq of catalyst.
Using this latter technique the compound containing the amide bond can be isolated with greater than 95% purity.
The method of the preferred embodiment typically results in greater than about 60% yield of compound containing an amide bond. As noted below, it has been observed that, in comparing otherwise identical amide formation reactions, the presence of the PP-HOBT catalyst made a significant improvement in the yield over that without its presence.